The order was given due to "high winds and fires threatening residential and commercial property," according to a statement from Schwarzenegger's office.
"The fires continue to expand and create a situation of extreme peril for the fire-fighters, the residents and the businesses of the county.
"The situation is beyond the capability and control of the county," it added.
The blaze was set early on Thursday by an arsonist in scrub forest east of Los Angeles, according to investigators and claimed the lives of four fire-fighters and left a fifth in critical condition.
Authorities said the fire was deliberately set in a desert region 200 kilometres (125 miles) east of Los Angeles, and whipped up by desert winds that later sent it shooting over a fire engine and its five-man crew.
Schwarzenegger also ordered flags at the state legislature to fly at half mast in recognition of those killed tackling the blaze.
Early on Friday, the fire was still being stirred up by strong winds after already ravaging nearly 5,000 hectares (nearly 12,000 acres) of land.
Some 1,200 fire-fighters were called in to tackle the blaze, which caused 700 people to be evacuated from their homes.
Local media have announced a 100,000-dollar reward for information leading to the arrest of the arsonist, who could face murder charges.
This year has been by far the worst for wildfires in the United States since records began in 1960, with some 38,000 square kilometres (around 14,500 square miles) of land going up in smoke so far, compared to some 35,000 hectares in 2005 -- until now a record year.

Copyright AFP (Agence France-Presse), 2006